A digital camera having an image synthesis function based on brightness is known. Such an image synthesis is to continuously shoot images in time series in the same composition under the same exposure, compare pixel output values of the same coordinates of a reference image and a comparison image, and synthesize the images while replacing the pixels of the reference image with the pixels of the comparison image with a higher output value or a lower output value. By this technique, for example, in photographing night scene, images of star tracks in a background with proper exposure can be captured. Digital cameras with such brightness-based image synthesis function are already available.
Conventionally, for brightness-based image synthesis continuous images are generated with a camera and the images are synthesized by image processing software with a personal computer after shooting. Therefore, a user cannot generate synthesized images at the time of photographing. By use of a digital camera having image synthesis function, a user can check synthesized images during shooting.
Further, for image synthesis with a personal computer, used image data are, for example, JPEG images with noise reduced and compressed when needed, therefore, the quality of the image data may not be good enough for image synthesis depending on photographic setting. Meanwhile, a digital camera can synthesize RAW images so that it can capture minute stars in colors which are likely to be considered as noise in a single frame, and improve image quality by averaging random noises in synthesizing process, which is not feasible by general image processing.
Meanwhile, there is a problem in the synthesis of RAW images with the occurrence of noises from which the JPEG images are free because RAW images are raw output data from an image sensor not subjected to image processing. Due to shot noises or random noises, even image data of the same subject captured at the same setting cannot be the same output and is an output with probability distribution. While repeating a simple brightness-based image synthesis on pixels at the corresponding coordinates, the outputs gradually approach the maximal value of the distribution, resulting in generation of defective images such as over-exposure images or magenta-like images. This problem does not occur with JPEG images subjected to a digital clamp or noise reduction.
Japanese Patent No. 4243084 (Reference 1) and Japanese Patent Application Publication No. 2010-34845 (Reference 2) disclose an imaging device to deal with failures such as a misadjustment or unevenness of a black level, for example. Reference 1 discloses an imaging device to adjust clamping to RAW images in accordance with parameters for photographic conditions so as to reduce a variation in black level. It generates clamp signals properly in accordance with exposure data or temperature data in long exposure shooting or under a high ambient temperature. Reference 2 discloses a technique to tentatively increase an OB signal defining the black level in clamp processing to stabilize the black level, thereby preventing degradation in image quality due to a variation in the black level. However, the techniques in the two references cannot deal with variation or unevenness in the black level caused by brightness-based image synthesis of RAW images.
As described above, in synthesizing RAW images with an imaging device such as a digital camera having a brightness-based image synthesis function, a variation, unevenness or misadjustment in the black level may occur in accordance with a photographic condition under which RAW images are captured, an ambient condition, or a synthesis condition such as the number of images synthesized.